Apparatus for electric signaling systems



Sept. 1933- L. D. WHITELOCK 2,129,313

APPARATUS FOR ELECTRIC SIGNALING SYSTEMS Filed July 1, 1955 I Q 24 Carrier fiequency 26 Generator To Source of ,5 INVENTOR Leland D z'zehclc. BY S K HIS ATTORNEY Patented Sept. 6, 1938 UNITED STATES PATENT OFFIE APPARATUS FOR ELECTRIC SIGNALING SYSTEMS Application July 1, 1935, Serial No. 29,338

2 Claims.

My invention relates to apparatus for electric signaling systems, and particularly to apparatus for carrier systems of signaling.

Balanced circuits have been proposed for signaling systems when it is desired to suppress or balance out certain frequencies of a periodic signaling current. For example, balanced circuits are provided for a push-pull type of amplifier for balancing out even harmonics of a signaling current. Again, balanced modulator and demodulator circuits are employed where the carrier frequency voltage is fed in phase to the control grids of two electron tubes, while the signaling voltage is fed to the grids 180 out of phase, and the carrier is suppressed in the output circuits of the tubes leaving only side band frequencies to be transmitted. In this latter case, in order to have the carrier suppressed to a minimum, the circuits must be perfectly balanced. Previous 20 methods of balancing electron tube circuits have used a combination of resistors and condensers in either the input or output, or both, of the electron tubes. In such methods heretofore employed variations in tube characteristics require readjustments of the balancing units after a change of a tube, and such adjustments may be quite critical since the balancing capacities are connected across the large impedances of the circuits. Adjustments of the balanced circuits being critical, changes in temperature and humidity may seriously affect the balance with the result that satisfactory operation may not be accomplished.

A feature of my invention is the provision of apparatus for signaling systems which compensate for differences in input and output circuit characteristics of vacuum tube circuits, particularly in connection with balanced vacuum tube modulators and demodulators employed in suppressed carrier systems such as, for example, suppressed carrier telephone, systems. A further feature of my invention is the provision of apparatus of the type here involved which is simple in construction, easily adjusted, not critical in adjustment and which in effect alters the vacuum tube characteristics to match up with the circuit conditions, and substantially perfect balanced conditions are obtained. Other features and advantages of my invention will appear as the specification progresses. For a better understanding of my invention, reference may be had to the accompanying drawing wherein the invention is disclosed in connection with vacuum tube circuits operative to modulate or demodulate a carrier frequency current. It will be understood, of course, that apparatus embodying my invention is equally useful for other balanced vacuum tube circuits.

I will describe two forms of apparatus embodying my invention, and will then point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 is a diagrammatic view of one form of apparatus embodying my invention, and Fig. 2 is a diagrammatic View of a second form of apparatus which also embodies my invention. Fig. 3 is a diagram wherein condensers are substituted for the interelectrode capacities of the elements of the electron tubes of Figs. 1 and 2 to better illustrate the bridge action of the balanced circuits of my invention.

In each ofthe different views like reference characters designate similar parts.

Referring to Fig. 1, two electron tubes 4 and 5 are provided with balanced circuits wherewith H signaling voltages are impressed on the grids 180 out of phase and a carrier frequency voltage is impressed on the grids in phase and suppression of the carrier frequency is secured by a push-pull arrangement of the output transformer windings interposed in the plate circuits of the tubes. As here shown, the tubes 4 and 5 are of the indirectly heated cathode type, the filaments of these two tubes being heated by a battery 6. The grids l and 8 of the respective tubes 4 and 5 are connected with the outside terminals of the secondary windings 9 and i6, respectively, of an input transformer TI, and the inside terminals of these two windings are joined and connected to the two cathodes l l and E2 of the tubes and 5 in parallel, a resistor l3 and a biasing unit I l being interposed in this latter connection. The biasing unit l4 comprises a rheostatic type potentiometer which includes a resistor i5 and a sliding contact l6, and two condensers I! and I8 connected in series across the resistor l5. As here shown, the condenser I8 is preferably adjustable while the condenser l'l may be a fixed condenser. A ground electrode I9 is connected between the sliding contact l6 and the adjacent terminal of resistor l3 as will be understood by an inspection of Fig. 1.

The plates 20 and 2! of the respective tubes 4 and 5 are connected with the outside terminals of the primary windings 22 and. 23 of an output transformer T2, and the inside terminals of these two windings are joined and connected over wire 25 to the positive terminal of a source of plate voltage such as a generator 24, the negative terminal of which is connected with a ground electrode 26. Hence, a plate circuit can be traced for the tube 4 from the positive terminal of generator 24 over wire 25, primary winding 22 of transformer T2, plate 20 of tube 4, intervening tube space tocathode II, upper portion of resistor I5 to the sliding contact I6, and thence over ground electrodes I9 and 25 to the negative terminal of the generator 24. A similar plate circuit for the tube 5 includes the positive terminal of generator 24, wire 25, primary winding 23 of transformer T2, plate 2I of tube 5, intervening tube space to the cathode I2, the lower portion of resistor I5 to the sliding contact l6, and thence over ground electrodes I9 and 26 to the negative terminal of generator 24'.

It is clear that the plate current for the tube 4 creates a voltage drop between the upper terminal of resistor I5 and the sliding contact I6 which voltage constitutes a biasing voltage for the grid cathode circuit of that tube, and the plate current for the tube 5 creates a voltage drop between the lower terminal of the resistor I5 and the contact 55 which constitutes a biasing voltage for the grid cathode circuit of tube 5. Hence the biasing voltages provided for the two tubes may be of similar, but not identical, magnitude by adjusting the position of the contact I6. Separate biasing voltages for the tubes 4 and 5 allow the tubes to be of slightly different characteristics and the differences in the amplification factor adjusted to produce a balanced output condition.

A blocking condenser 2'! is connected between the inside terminals of the primary windings 22 and 23 of transformer T2 and the inside terminals of the two condensers I1 and I8, while a ground electrode 28 is connected between the condenser 2's and the inside terminals of the condensers H and I8 as will be readily understood by an inspection of Fig. 1. It follows that the condenser Il serves as a by-pass for alternating current around the upper portion of the resistor I5, that is, around the grid cathode biasing resistor of tube 4. The condenser I8 serves as a by-pass around the lower portion of resistor I5 which is the biasing resistor for tube 5. Since the condenser I8 is made adjustable, differences in the inter-electrode tube capacities and of the input and output circuit capacities may be readily compensated by varying the value of condenser I8 with respect to condenser I'I. Consequently, the condensers H and I8 each serve the dual function of balancing the capacities of the tubes and. the circuits, and of by-passing the alternating currents around the biasing resistors.

A carrier frequency generator 29 is connected by wires 55 and 3I across the resistor I 3 which.

is interposed in the grid cathode circuits for the tubes 4 and 5 as pointed out hereinbefore. The generator 25 may be of any convenient type. among them being the vacuum tube type, and the frequency of the carrier current supplied by the generator 29 may be selected as desired. It follows that the carrier voltage drop across the resistor I3 is impressed in phase on the grids I and 8 and of equal magnitude providing the secondary windings 9 and I0 of transformer TI are approximately equal. The signaling voltage which is to be impressed upon the carrier for modulation or which is to be mixed with the carrier for demodulation is supplied over wires 32 and 33 to the primary windings 34 and 35 of transformer TI, the two windings 34 and 35 being preferably connected together to serve as one continuous winding. Consequently the electromotive forces induced in the secondary windings 9 and I0 due to the signaling current flowing in the primary windings 34 and 35 are impressed upon the grids l and 8 of tubes 4 and 5 substantially out of phase. The addition and subtraction of the carrier voltage and the signaling voltage when thus impressed on the grids of the tubes 4 and 5 produce in the plate circuits resultant side band frequencies and a carrier frequency component, the latter of which is substantially of equal magnitude in the plate circuits of each of the two tubes since the biasing unit I4 through the setting of the contact I6 is effective to establish a balanced condition of the amplification of the two tubes and since the adjustment of the condenser I8 is effective to establish a balanced condition of the tube and circuit capacities. The carrier frequency component being of substantially equal magnitude in each of the plate circuits, it is suppressed to substantially a zero value by the differential arrangement of the windings of the output transformer. Furthermore, such balanced conditions of the circuits effects in the output circuit an addition of the side band frequencies with little or no distortion.

The two secondary windings 35 and 37 of the output transformer T2 are connected together for adding their effects and the electromotive forces induced therein are supplied over wires 33 and 39 to any desired apparatus which in the case of modulation would ordinarily include a transmitting circuit and in the case of demodulation would ordinarily include a signaling device.

Fig. 3 is an equivalent diagram where condensers representing inter-electrode capacities are substituted in place of the tube elements. In Fig. 3, the reference characters Cop represent the grid plate capacities, the reference characters CGo the grid cathode capacities, and the reference characters Ccp the cathode plate capacities of the tubes 4 and 5. It is easily to be seen that slight differences between the resistance components of the input and output circuits may be regulated by varying the position of the slidable contact of the resistor I5. Also, small differences in the amplification factor of the tubes may be equalized by adjusting the contact I6. Furthermore, variations in inter-electrode tube capacities may be easily compensated by changing the value of condenser I8 with respect to the condenser ll. It should be noted, of course, that windings 9 and I0 of transformer TI should be approximately equal and likewise the windings 22 and 23 of the transformer T2 should be equal, in order for the balance obtained by the sliding contact I 5 to be effective without greatly changing the bias on one tube.

With apparatus constructed in the manner described for Fig. 1, I have found that once the relative value of condenser IS with respect to the condenser IT has been set for a given input and output circuit condition, then that adjustment usually does not have to be changed with a change of a tube. The initial adjustment of condenser I8 is quite simple as the capacity of this condenser is relatively large and hence its adjustment is not critical. Furthermore, I have found that often the position of the sliding contact I6 on resistor I5 does not have to be changed following a change of a tube, and if it does the change is usually very slight and a perfect balanced condition is easily and quickly obtained. Since the adjustments are not critical, I have found that changes effected due to changes in temperature and humidity do not materially change the balanced conditions of the circuits.

The apparatus of Fig. 2 is the same as disclosed in Fig. 1 except for the fact that a resistor 40 is added to the biasing element [4, and this resistor 40 is interposed between the sliding contact I6 and the ground electrode I9. The biasing voltage for the tube 4 is hence the voltage drop between the top terminal of resistor I5 and the left-hand terminal of resistor 4!], that is, the terminal adjacent the ground electrode I9, and the biasing voltage for the tube 5 is the voltage drop between the lower terminal of resistor l5 and the left-hand terminal of resistor 40. It is clear that a balanced condition of the circuits is readily obtained by the apparatus of Fig. 2 in a manner similar to that explained in connection with the apparatus of Fig. 1.

As pointed out hereinbefore this method of compensation for tube characteristics and circuit conditions can be applied to any type of push-pull circuits and its use is not confined solely to the type of circuits herein illustrated where both a signaling frequency current and a carrier frequency current are fed to the grids of modulator and demodulator tubes.

Although I have herein shown and described only two forms of apparatus embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination; two electron tubes each having a plate, a grid, and a cathode; an input transformer the two outside secondary winding terminals of which are connected one to the grid of one tube and the other to the grid of the other tube and its central terminal connected to a common ground electrode, an output transformer the two outside primary winding terminals of which are connected'one to the plate of one tube and the other to the plate of the other tube, a source of plate voltage connected between a central terminal of said primary winding and said ground electrode, a first resistor having a slidable contact as well as two outside terminals which are connected one to the cathode of one tube and the other to the cathode of the other tube for eifectively balancing the internal tube resistances by virtue of the position of said contact, two condensers serially connected between the cathodes of said tubes and having their mid terminal connected to said ground electrode and at least one of which is adjustable for effectively balancing the inter-electrode tube capacities, and a second resistor connected between said contact and ground electrode for effectively biasing the grids of said tubes with respect to the cathodes.

2. In combination; two electron tubes each having a plate, a grid, and a cathode; an input transformer the two outside secondary winding terminals of which are connected one to the grid of one tube and the other to the grid of the other tube and its central terminal connected to a common ground electrode, an output transformer the two outside primary winding terminals of which are connected one to the plate of one tube and the other to the plate of the other tube, a source of plate voltage connected between a central terminal of said primary winding and said ground electrode, a first resistor having a slidable contact as well as two outside terminals which outside terminals are connected one to the cathode of one tube and the other to the cathode of the other tube, means including a second resistor connected between said contact and common ground electrode to balance the internal tube resistances by virtue of the position of said contact and to effectively bias the grids of the tubes with respect to the cathodes, two condensers serially connected between the cathodes of the tubes and one of which condensers is adjustable, and means to connect the mid terminal of said condensers with said ground electrode to balance the inter electrode tube capacities by virtue of the adjustable condenser and to serve as a by-pass around said first and second resistors.

LELAND D. WHITELOCK. 

